FIGS. 1 and 2 show a conventional motor structure disclosed by U.S. Pat. No. 6,654,213, which includes a housing 70, a rotor 80, and a stator 90. Referring to FIG. 1, the housing 70 includes a hollow shaft tube 71 receiving a bearing 72. The shaft tube 71 has an opening 711 at an upper end thereof for the bearing 72 to be inserted into the shaft tube 71, while the shaft tube 71 peripherally has a support face 712 facing upwards. The rotor 80 includes a shaft 81 rotatably extending through the bearing 72. The stator 90 is mounted around the shaft tube 71 and includes an upper bobbin 91, a lower bobbin 92, a plurality of silicon steel plates 93 sandwiched between the upper and lower bobbins 91 and 92, and a winding 94 wound around the silicon steel plates 93. The upper bobbin 91 includes an abutting portion 911 in the form of an annular ring extending inwards radially to block an outer rim of the opening 711, preventing disengagement of the bearing 72 from the shaft tube 71 via the opening 711. The bearing 72 and shaft tube 71 are in loose coupling with each other and have a small gap therebetween to allow easy assembly and to prevent the shaft tube 71 from being squeezed and thus deformed by the bearing 72. Furthermore, the support face 712 of the shaft tube 71 faces and abuts against the silicon steel plates 93 so as to support and position the stator 90 relatively to the housing 70.
However, referring to FIG. 2 showing a cross sectional view of the conventional motor structure while the housing 70, rotor 80, and stator 90 are assembled, it is apparent that, in an axial direction along the shaft tube 71, a first distance between the abutting portion 911 and a lower end of the plurality of silicon steel plates 93 must be equal to a second distance between the opening 711 and support face 712, or the stator 90 cannot surely rest on the support face 712 if the first distance is shorter than the second distance, or the abutting portion 911 is unable to firmly block the outer rim of the opening 711 if the first distance is longer than the second distance. Therefore, the accuracy in sizes of the housing 70 and stator 90 is extremely required. Similarly, since the abutting portion 911 merely extends inwards in radial directions of the shaft tube 71 to block the outer rim of the opening 711, a top end of the bearing 72 must just locate at the opening 711 for the abutting portion 911 to abut against. It should be noted that all of the above requirements have to be extremely fulfilled, or damages to the stator 90, especially to the abutting portion 911, or axial vibration of the bearing 72 will be caused. Accordingly, the manufacture cost of this conventional motor structure is high, in particular to those applied to miniature motors, for exactly making the housing 70, rotor 80 and stator 90 by precision machines and processes. As a result, it is necessary to improve the conventional motor structure for a lower cost of manufacture and a desirable positioning performance.